Unfortunately, interest rates are at an historic low so "letting it grow" isn't a realistic option. Basically you will get next to nothing in interest and then the government will tax it (except for the State Savings of course - which makes it by far the best buy). Any "growth" is limited to start with, taxed to the hilt and wiped out by inflation to finish with.
So if you put €10000 into State Savings (to get the best rate), after 5 years you would have the princely sum of €10,500. €500 is not to be sneezed at but would you realistically want to tie up that €10000 for 5 years?
if you wanted to have money you could access quickly, you would find the rates are even worse.
Higher returns are really only accessible with more speculative investments and those are much riskier and far from guaranteed.
So while it would be prudent not to splurge your windfall, don't bank on letting it grow either. Hold some for a rainy day and put some to work for things that you need first and want second. The person who willed you the money trusted you to choose what is best for you, trust yourself too.